1. Field of the Invention
The present invention relates to an optical receiving apparatus that receives an optical signal modulated in a differential phase shift keying (DPSK) modulation format.
2. Description of the Related Art
In recent years, the demand for introducing a next-generation 40 Gb/s optical transmission system has been increasing that also achieves transmission distance and spectrum efficiency equivalent to a 10 Gb/s system. As a method for implementing the system, research and development have become active on the return-to-zero DPSK (RZ-DPSK) modulation format or the carrier suppressed RZ-DPSK (CSRZ-DPSK) modulation format that is superior in terms of an optical signal noise ratio (OSNR) tolerance and nonlinearity tolerance compared to the non return-to-zero (NRZ) modulation method conventionally applied in a system of 10 Gb/s or less.
In addition to the modulation formats mentioned above, research and development have become active also on phase modulation formats with a narrow spectrum (high frequency), such as the RZ differential quadrature phase shift keying (RZ-DQPSK) modulation format or the CSRZ-DQPSK modulation format. As for the optical receiving apparatus that demodulates the optical signal modulated with the DPSK modulation format, the optical receiving apparatus using a delay interferometer has been studied (for example, Japanese Patent Application Laid-open Publication No. 2004-516743).
However, if 40 Gb/s or 43 Gb/s transmission is performed by the optical receiving apparatus that utilizes the modulation formats mentioned above, wavelength dispersion tolerance would be reduced to approximately 1/16 of that of 10 Gb/s transmission. For this reason, it is necessary to arrange a variable chromatic dispersion compensator (VDC) at a receiving end of the optical receiving apparatus to perform dispersion compensation with high precision.
In this case, the optical receiving apparatus requires not only to control a setting value of a phase shift amount for the delay interferometer, but also to control the setting value of a dispersion compensation amount for the variable chromatic dispersion compensator. In other words, upon receiving the optical signal modulated with the (CS) RZ-D(Q)PSK modulation format, it is required to optimally set both the delay interferometer and the variable chromatic dispersion compensator for demodulating the received optical signal.
As for dispersion compensation, it is possible to monitor an error condition using the number of error corrections for the decoded received signal and control the variable chromatic dispersion compensator in accordance with the monitored error condition. However, the dispersion compensation amount in the variable chromatic dispersion compensator and the phase shift amount in the delay interferometer have different properties for the number of errors from each other. For this reason, it is required to search for the optimal values for both the dispersion compensation amount and the phase shift amount, or a combination thereof, to improve the quality of the received signal, leading to a problem that it takes time until the control of the variable chromatic dispersion compensator and the delay interferometer is stabilized.
FIG. 17 is a graph of the dispersion compensation amount and a phase shift, associated with a power penalty. The dispersion compensation amount represents the dispersion compensation amount in the variable chromatic dispersion compensator. The phase shift represents the phase shift in the delay interferometer. The power penalty represents an amount of increase in received optical power required to obtain a desired bit error rate (BER), and the power penalty decreases as the BER decreases. As shown in FIG. 17, the dispersion compensation amount where the power penalty is the minimum (i.e., the BER is the minimum) varies depending on the phase shift.
FIG. 18 depicts process of searching and setting the dispersion compensation amount and the phase shift amount. Since the dispersion compensation amount where the BER is the minimum varies depending on the phase shift as described above, it is required to change the dispersion compensation amount and the phase shift amount alternatively while monitoring the BER, as shown in FIG. 18, to search for the combination of the dispersion compensation amount and the phase shift amount where the BER is the minimum. Therefore, it takes time (for example, approximately 10 minutes) until the dispersion compensation amount and the phase shift amount are optimally set to stabilize the optical receiving apparatus when the optical receiving apparatus is started up, protected, or a communication path thereof is switched.
In addition, since wavelength dispersion in a transmission line or an optical phase difference in the delay interferometer varies depending on a temperature change or the like during operation of the optical transmission system, it is required to set the dispersion compensation amount and the phase shift amount so as to follow the variation. However, it takes time to search for the optimal dispersion compensation amount and the optimal phase shift amount, as described above. Therefore, the setting that follows the variation cannot be achieved if these amounts are searched whenever a wavelength dispersion amount in the transmission line or the optical phase difference varies.